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Prototype for testing: The concrete hollow sphere was manufactured at a 1:10 scale and has a three-metre diameter.
© Fraunhofer IWES / Energiesystemtechnik
StEnSea pumped storage power plant

The schematic shows the principle behind the StEnSea sphere-based pumped storage system.
© Hochtief Solutions

Concrete sphere for the sea successfully stores electricity

How can energy be stored near offshore wind farms in the future? Researchers have developed a new storage system that works with hollow spheres on the seabed. The principle is similar to conventional pumped storage power plants. At the end of 2016, the researchers at Fraunhofer Institute for Wind Energy and Energy System Technology IWES tested a three-metre concrete prototype in Lake Constance. The test results are highly promising.

How can the enormous amounts of electricity generated by offshore wind power be temporarily stored on site? Researchers from the Fraunhofer Institute for Wind Energy and Energy System Technology IWES have now developed a new concept to achieve this. In the "Stored Energy in the Sea" (StEnSea) project, the scientists have constructed a pumped storage power plant that uses the sea itself as the upper storage reservoir. Hollow spheres on the bottom of the ocean serve as the lower reservoir. If more wind power is generated than required, it is used to empty the spheres by pumping. When there is a demand for electricity, high-pressure water flows back into the sphere via a turbine. The turbine drives a generator that feeds the energy stored in the sphere into the grid as electricity. This principle will enable large storage capacities to be installed in future in the vicinity of offshore wind farms.

After several years of research, the first prototype went into the testing phase at the end of November 2016. The researchers tested the concrete hollow sphere in Lake Constance for one month. The 1:10 scale model with a diameter of about three metres was lowered to a depth of 100 metres about 200 metres offshore. During the four-week test, the researchers investigated detailed aspects concerning the construction, installation, design of the powertrain and electrical system, the operation management and control, the status monitoring as well as the dynamic modelling and simulation of the overall system. The results are consistently positive. "We actually managed to successfully save energy and run a variety of different cycles. It worked exactly as we had imagined," says StEnSea project manager Matthias Puchta.

The deeper, the better

In large water depths the hollow spheres can utilise the high water pressure. The storage capacity for a 30-metre sphere at 700 metres is about 20 megawatt-hours. It increases linearly with the water depth for a given volume. But the ratio of the wall thickness of the concrete sphere to the water pressure also plays a role, explains the project manager: "In about 750 metres of water depth, the external pressure means that the wall thickness has to exactly correspond to how much the sphere has to weigh anyway so that it stands securely on the seabed without anchoring."

With the results of the pilot experiment, the research team wants to more closely investigate suitable locations for a demonstration project in Europe more closely. They are aiming for a sphere diameter of 30 metres for the system's demonstration scale. This is the currently sensible target size in technical and economic terms, given the underlying engineering conditions. There is considerable potential for using sea-based pumped storage systems at offshore sites, especially off the coasts of densely populated regions such as in the Norwegian Trench. The EU has the greatest potential, followed by the USA and Japan.

Project manager Matthias Puchta in interview

In the interview, project manager Matthias Puchta from Fraunhofer IWES talks in detail about the successful test in Lake Constance and about the cost estimates. He is optimistic about all the places where the new storage system can be deployed. The full interview is available at forschung-energiespeicher.info.



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